Joining rooms made with archimesh creates one sided model
Archimesh uses a so called solidify-modifier to make your walls thick (instead of "real" geometry that you can manipulate in edit mode)

you have multiple options to change the materials:


you can apply the modifier: go to the panel on the right and click on the blue wrench icon. there you can click apply (green box in the screenshot)
you can enter a material index offset of "1" in the modifier panel and simply add a second material to you walls